Heat the oil in a wok over medium-high heat. Add mustard seeds and cook for 30 seconds or until they start to pop, then add the curry powder, turmeric, ginger, chilli, curry leaves, cardamom and garlic. Cook, stirring, for 1-2 minutes until fragrant. Add onion and 1 teaspoon salt and cook, stirring, for a further 1-2 minutes until onion has softened. Add mango and tomato, continue to cook for a further minute, then stir in the coconut milk. Cook over low heat for 1-2 minutes just to warm through.
Stir through coriander and cashews, then serve the curry with steamed rice.
From selected greengrocers.